1. Field of the Invention
The present invention relates to a radio reception apparatus, a radio reception method and a radio reception program. More specifically, the present invention relates to a radio reception apparatus, a radio reception method and a radio reception program that are capable of switching a plurality of modulation methods having different multi-value numbers in accordance with quality of communication.
2. Description of the Background Art
Conventionally, in a mobile communication system such as PHS (Personal Handyphone System), communication between a mobile terminal (hereinafter referred to as a terminal) and a radio base station (hereinafter referred to as a base station) is established by using a prescribed modulation method, for example, using well-known QPSK (Quadrature Phase Shift Keying) modulation method.
FIG. 10A shows an arrangement of symbol points in accordance with the QPSK modulation method on an IQ coordinate plane. Referring to FIG. 10A, according to the QPSK method, a symbol point of a received signal corresponds to any of the four signal points positioned concentrically on the IQ coordinate plane. Therefore, it is possible to transmit at one time 2 bits of data representing any of the four signal points. Conventionally, communication between the terminal and the base station is performed using a fixed modulation method, for example, the QPSK modulation method described above.
Recent mobile communication systems, however, require data transmission of larger volume at higher speed such as data communication, as compared with conventional voice communication. Accordingly, multi-value modulation methods having number of multi-value larger than the QPSK method mentioned above have been developed. As an example of such multi-value modulation method, 16QAM (Quadrature Amplitude Modulation) method has been known and practically utilized in some systems of data communications.
FIG. 10B shows an arrangement of symbol points in accordance with 16 QAM modulation method on the IQ coordinate plane. Referring to FIG. 10B, according to 16QAM modulation method, a symbol point of a received signal corresponds to any of a total of 16 signal points on the coordinate plane, arranged four by four in a lattice form in each quadrant of the IQ coordinate plane. Therefore, it is possible to transmit at a time 4 bits of data representing any of 16 signals.
When a modulation method having a larger multi-value number such as the 16 QAM method is employed as a method of modulation in a mobile communication system such as the PHS and communication environment of the propagation path is defective (if the propagation path has severe interference or noise), then symbol points may possibly be recognized erroneously, because the interval between each of the symbol points is narrow and symbol points are arranged tightly in 16 QAM, as can be seen from the arrangement of symbol points of FIG. 10B. Therefore, though this method has communication speed faster than the QPSK modulation method shown in FIG. 10A, it is more prone to make reception errors.
In view of the foregoing, a concept of adaptive modulation has been proposed, in which communication is performed while adaptively switching between a modulation method having a smaller multi-value number such as the QPSK (which is slower in communication speed but less susceptible to the influence of propagation path) and a modulation method having a larger multi-value number such as 16 QAM (which is faster in communication speed but more susceptible to the influence of propagation path) in consideration of the state of the propagation path, that is, the quality of communication (reception), in order to improve speed of communication as much as possible.
Specifically, an approach has been proposed in which the quality of communication (reception) of the propagation path is evaluated using some parameter, and the multi-value number is increased from QPSK to 16 QAM, for example, to improve the speed of communication only when a prescribed quality is satisfied.
Conventionally, reception level, reception error (for example, FER: Frame Error Rate) and interference level (for example, CIR: Carrier to Interference Ratio, representing a ratio of the desired wave and the interference) have been considered as parameters for evaluating the communication quality of the propagation path.
The aforementioned parameters to be the basis of switching among modulation methods proposed in the conventional method of adaptive modulation, however, are not always appropriate for evaluating the communication quality of the propagation path.
Specifically, the magnitude of reception level increases when a radio apparatus of the counter part of communication comes closer. Therefore, it is not the case that a higher reception level means a good communication environment (with small interference or noise) of the propagation path. Further, as can be seen from the comparison of the modulation methods shown in FIGS. 10A and 10B, even when there is no reception error with a modulation method (QPSK) having smaller multi-value number (smaller number of symbol points on the IQ plane), it is unpredictable whether there arises reception error or not with another modulation method (16QAM) having a larger multi-value number (having dense symbol points on the IQ plane). Meanwhile, it requires special and complicated procedure and technically very difficult to actually measure the interference level (for example, CIR) of the propagation path during communication.
Further, different radio reception apparatuses employ different methods of reception (for example, a conventional reception method with one antenna, an adaptive array reception method with plural antennas). In addition, performances and qualities of components such as filters used in the reception apparatuses vary one by one, and such differences and variations have influence on the quality of communication.
The conventional parameters mentioned above, however, do not reflect such quality or performances of the reception apparatuses. From this point also, it has been difficult to exactly evaluate the communication quality by using the conventional parameters.
Therefore, in the conventional adaptive modulation technique, it has been difficult to correctly switch among modulation methods having different multi-value numbers while exactly evaluating the communication quality of the propagation path.